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JAVA, JAVA, JAVA
Object-Oriented Problem Solving
Third Edition
Ralph Morelli | Ralph Walde
Trinity College
Hartford, CT
published by Prentice Hall
Java, Java, Java
Object Oriented Problem Solving
Chapter 7: Strings and String
Processing
Objectives
• Be more familiar with Java Strings.
• Know how to solve problems that involve
manipulating strings.
• Be able to use loops in designing string
processing algorithms.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Outline
•
•
•
•
•
•
•
•
•
•
•
Introduction
Basics of the Java String Type
Finding Things within a String
Example: Keyword Search
From the Java Library: StringBuffer
Retrieving Parts of Strings
Example: Processing Names and Passwords
Processing Each Character in a String
Comparing Strings
From the Java Library: StringTokenizer
Handling Text in a Graphics Context (Optional)
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Introduction
• A data structure is a collection of data that
is organized (structured) in some way.
• A string is a collection of character (char)
data. Strings are important data structures in
a programming language
• Strings are used to represent a wide variety
of data.
• In Java, the Object.toString() method
represents an object as a string.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
String Basics
• A java.lang.String object is a sequence of
characters plus a collection of methods for
manipulating strings.
• Unlike other Java objects, Strings have certain
characteristics in common with the primitive data
types.
• For example, strings can have literals. A String
literal is a sequence of zero or more characters
contained in double quotes -- for example,
“Socrates” and “” (empty string).
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
The String Class
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Constructing Strings
• The String constructors:
public String();
public String(String initial_value);
// Creates an empty string
// Creates a copy of a string
• The following constructor statement creates a
String object and makes name a reference to it:
String name = new String();
String instance variables.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Constructing Strings (cont)
• A literal -- e.g., “Socrates” -- is considered a
reference to a String object. All occurrences of
“Socrates” in a program refer to the same object.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Constructing Strings (cont)
• When literals are assigned to String variables Java
makes those variables serve as references to the
literals.
String name1 = "";
String name2 = "Socrates";
String name3 = "Socrates";
// Reference to the empty string
// References to "Socrates"
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Constructing Strings (cont)
• New String objects are created whenever the
String constructors are used:
String name4 = new String();
String name5 = new String("Socrates");
String name6 = name4;
Java, Java, Java, 3E by R. Morelli | R. Walde
// Creates an object
Copyright 2006. Chapter 7: Strings
Concatenating Strings
• When surrounded on either side by a String, the +
symbol is used as a binary concatenation operator.
It has the effect of joining two strings together.
String lastName = "Onassis";
String jackie = new String("Jacqueline " + "Kennedy " + lastName);
“Jacqueline Kennedy Onassis”
• Primitive types are automatically promoted to
strings when mixed with concatenation operators.
System.out.println("The square root of 25 = " + 5);
Output: The square root of 25 = 5
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Indexing Strings
• The number of characters in a string is its length.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
// string1.length() ==> 0
"Hello";
// string2.length() ==> 5
"World";
// string3.length() ==> 5;
string2 + " " + string3; // string4.length() ==> 11;
• The position of a character within a string is called
its index. Strings are zero indexed -- the first
character is at index 0.
Note: Because of zero
indexing, the last
character in a string of 8
characters is at index 7.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Converting Data to String
• The String.valueOf() methods are class methods
that convert primitive types into String objects.
static public String valueOf( primitive type
String
String
String
String
number = new String (String.valueOf(128));
truth = new String (String.valueOf(true));
bee = new String (String.valueOf('B'));
pi = new String(String.valueOf(Math.PI));
Recall that one refers to
class methods by using the
class name as the qualifier.
Java, Java, Java, 3E by R. Morelli | R. Walde
//
//
//
//
);
Creates
Creates
Creates
Creates
"128"
"true"
"B"
"3.14159"
Note the difference
between ‘B’ and “B”
Copyright 2006. Chapter 7: Strings
Finding Things within a String
• The indexOf() and lastIndexof() methods are
instance methods that can be used to find the index
position of a character or a substring within a
String.
public
public
public
public
int
int
int
int
indexOf(int character);
indexOf(int character, int startingIndex);
indexOf(String string);
indexOf(String string, int startingIndex);
public
public
public
public
int
int
int
int
lastIndexOf(int character);
lastIndexOf(int character, int startingIndex);
lastIndexOf(String string);
lastIndexOf(String string, int startingIndex);
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
IndexOf() and LastIndexOf()
• The indexOf() method searches from left to right
within a String for either a character or a substring.
• The lastIndexOf() method searches from right to
left for a character or substring.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
"Hello";
"World";
string2 + " " + string3;
string1.indexOf('o')
string2.indexOf('o')
string3.indexOf('o')
string4.indexOf('o')
==>
==>
==>
==>
-1
4
1
4
string1.lastIndexOf('o')
string2.lastIndexOf('o')
string3.lastIndexOf('o')
string4.lastIndexOf('o')
==>
==>
==>
==>
-1
4
1
7
A return value of -1 means
the character is not in the
string.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
IndexOf() and LastIndexOf()
• The indexOf() and lastIndexOf() methods can also
be used to find substrings, such as “or”.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
"Hello";
"World";
string2 + " " + string3;
string1.indexOf("or")
string2.indexOf("or")
string3.indexOf("or")
string4.indexOf("or")
==>
==>
==>
==>
-1
-1
1
7
string1.lastIndexOf("or")
string2.lastIndexOf("or")
string3.lastIndexOf("or")
string4.lastIndexOf("or")
==>
==>
==>
==>
-1
-1
1
7
A return value of -1 means that
“or” is not in the string.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Example: Keyword Search
• Finding a keyword within a string is a task that
word processors and browsers must do.
• Algorithm Design: Find every occurrence of some
keyword, K, within a string, S:
Suppose S is our string and K is the keyword.
Initialize a counter variable and result string.
Set P to the indexOf() the first occurrence of K in S.
While ( P != -1 )
While loop because there may
Increment the counter
be 0 or more occurrences.
Insert P into the result string
Set P to the next location of the keyword in S
Insert the count into the result string
Return the result string as a String
When P is -1, there are no
more occurrences of K in S.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Implementation: Keyword Search
/**
* Pre: s and keyword are any Strings
* Post: keywordSearch() returns a String containing the
* number of occurrences of keyword in s, followed
* by the starting location of each occurrence
Bound: When ptr is
*/
public String keywordSearch(String s, String keyword) {
-1, no more
String resultStr = "";
occurrences of s.
int count = 0;
int ptr = s.indexOf(keyword);
while (ptr != -1) {
Initializer
++count;
Updater
resultStr = resultStr + ptr + " ";
ptr = s.indexOf(keyword, ptr + 1);
// Find next occurrence
}
resultStr = count + ": " + resultStr;
// Insert the count
return resultStr;
// Return as a String
} // keywordSearch()
Test Performed
keywordSearch( "this is a test","is")
keywordSearch( "able was i ere i saw elba","a")
keywordSearch( "this is a test","taste")
Java, Java, Java, 3E by R. Morelli | R. Walde
Expected Result
2: 2 5
4: 0 6 18 24
0:
Test data should
test all possible
outcomes.
Copyright 2006. Chapter 7: Strings
Automatic Garbage Collection
• Immutability: Java Strings are cannot be modified.
Whenever you assign a new value to a String, Java must
create a new String object and discard the old one. In
resultStr = resultStr + ptr + “ “;
Java will create a new object (b) referenced by resultStr:
The original resultStr
object has no more
references to it so it has
to be garbage collected
which takes time.
Java, Java, Java, 3E by R. Morelli | R. Walde
This is the new
resultStr.
Copyright 2006. Chapter 7: Strings
Library: java.lang.StringBuffer
• Objects of java.lang.StringBuffer class are strings
that can be modified. Some of its methods are:
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
The Revised keywordSearch()
public String keywordSearch(String s, String keyword) {
StringBuffer resultStr = new StringBuffer(); // Create StringBuffer
int count = 0;
int ptr = s.indexOf(keyword);
while (ptr != -1) {
++count;
resultStr.append(ptr + " "); // Insert letters into it
ptr = s.indexOf(keyword, ptr + 1);
The revised
}
resultStr.insert(0, count + ": ");
keywordSearch()
return resultStr.toString(); // Convert buffer back to a String
uses a local
} // keywordSearch()
StringBuffer to
build the result, but
converts it back to
String before
returning.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Retrieving Parts of Strings
• The String class contains methods to retrieve
characters and substrings from a string.
Takes the
substring
from
startIndex
to endIndex
public char charAt(int index)
public String substring(int startIndex)
public String substring(int startIndex, int endIndex)
Takes the
substring from
startIndex to
the end of
string.
Note: endIndex points to index
after the last character taken.
String str = "HelloWorld";
str.substring(5)
==> "World"
str.substring(3)
==> "loWorld";
//
0123456789
String str = "HelloWorld";
str.substring(5,7)
==> "Wo"
str.substring(0,5)
==> "Hello";
str.substring(5, str.length())
==> "World"
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Example: Names and Passwords
• Problem: Suppose user names and passwords are
stored as delimited strings where ‘:’ is the delimiter.
Write methods to get the name and password.
• Algorithm: Use the indexOf()
smith:bg1s5xxx
mccarthy:2ffo900ssi
and substring() methods.
public String getName(String str) {
int posColon = str.indexOf(':');
// Find the delimiter
String result = str.substring(0, posColon); // Extract the name
return result;
These 3 could be a single statement:
}
return str.substring(0, str.indexOf(‘:’));
public String getPassword(String str) {
int posColon = str.indexOf(':');
// Find the delimiter
String result = str.substring(posColon + 1); // Extract the password
return result;
}
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Processing Each Character in a String
• For example, suppose you want to count the
number of occurrences of a certain character in a
string:
Use the string’s
length as a bound.
// countChar counts the number of ch’s in str
// Precondition: Neither str nor ch are null
// Postcondition: countchar() == the number of ch in str
public int countChar(String str, char ch) {
int counter = 0;
// Initialize a counter
for (int k = 0; k < str.length(); k++)
// For each character
if (str.charAt(k) == ch)
// If it's a ch
counter++;
//
count it
return counter;
// Return the result
}
Possible off-by-one Error: Remember that
the last character in a string is at index
length()-1.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Method Design: reverse()
• reverse() reverses the letters in its String
parameter.
A StringBuffer is used to
construct the reverse of s.
/**
* Pre: s is any non null string
* Post: s is returned in reverse order
*/
public String reverse(String s) {
StringBuffer result = new StringBuffer();
for (int k = s.length() -1; k >= 0; k--) {
result.append(s.charAt(k));
} //for
return result.toString();
} // reverse()
The result must be
converted back to a String.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Method Design: capitalize()
• capitalize() converts its parameter into a string
with an initial capital letter (Capitalize).
Note use of
toUpperCase(char)
method.
/**
* Pre: s is any non null string
* Post: s is returned with only its first letter capitalized
*/
public String capitalize(String s) {
if (s.length() == 0)
return s;
StringBuffer result = new StringBuffer();
result.append(toUpperCase(s.charAt(0))); // Change first to UPPERCASE
for (int k = 1; k < s.length(); k++) {
// Convert every other letter
result.append(toLowerCase(s.charAt(k)));
} //for
return result.toString();
} // capitalize()
private char toUpperCase(char ch) {
if ((ch >= ‘a’) && (ch <= ‘z’))
return (char)(ch - 32);
// Explicit cast required
return ch;
}
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Miscellaneous String Methods
• The String class contains additional useful string
manipulation methods, including the following:
Method Signature
boolean endsWith( String suffix)
boolean startsWith(String prefix)
String toUpperCase()
String toLowerCase()
String trim()
Example
"Perfection".endsWith("tion")
"Perfection".startsWith("Per")
"Perfection".toUpperCase()
"Perfection".toLowerCase()
" Perfection ".trim()
Value
true
true
"PERFECTION"
"perfection"
"Perfection"
• Note that methods such as toUpperCase(),
toLowerCase(), and trim() produce a new String
object because Strings cannot be modified.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Comparing Strings
• Strings are compared according to their
lexicographic order.
• Def: For strings s1 and s2, s1 precedes s2 in
lexicographic order if its first character precedes
the first character of s2. If their first characters are
equal, then s1 precedes s2 if its second character
precedes the second character of s2, and soon.
Finally, the empty string is handled as a special
case, preceding all other strings.
• The following strings are arranged in lexicographic
order:
" " "!" "0" "A" "Andy" "Z" "Zero" "a" "an" "and" "andy" "candy" "zero"
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Lexicographic Order
• A Java method that implements our
definition of lexicographic order.
public boolean precedes(String s1, String s2) {
int minlen = Math.min(s1.length(), s2.length()); // Pick shorter length
int k = 0;
// Start at the first character
while (k < minlen) {
// While more characters
if (s1.charAt(k) < s2.charAt(k))
// If kth s1 < kth in s2
return true;
//
then s1 precedes s2
else if (s2.charAt(k) < s1.charAt(k))
// If kth in s2 < kth in s1
return false;
//
s1 does not precede s2
else
// If neither case
k++;
//
go on to the next character
} // while
return s1.length() < s2.length();
// If all characters so far are equal
} // precedes()
//
s1 < s2 if it is shorter than s2
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
String Identity vs. String Equality
• Methods for comparing strings:
public boolean equals(Object anObject); // Overrides Object.equals()
public boolean equalsIgnoreCase(String anotherString)
public int compareTo(String anotherString)
• Two strings are equal if they have the same letters
in the same order:
String s1 = "hello";
String s2 = "Hello";
s1.equals(s2)
// false
s1.equals("hello”); // true
• Error: Using == to compare two strings. For
objects, o1 == o2 means o1 and o2 are identical.
• The == operator is equivalent to Object.equals()
method: o1.equals(o2) means o1 and o2 are
identical.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
String Identity vs. String Equality (cont)
Given the following declarations
String
String
String
String
String
String
s1
s2
s3
s4
s5
s6
=
=
=
=
=
=
new String("hello");
new String("hello");
new String("Hello");
s1;
// s1 == s4
"hello";
"hello";
We get the following equality results
s1.equals(s2)
s1.equals(s3)
s1.equalsIgnoreCase(s3)
s1.equals(s4)
s1.equals(s5)
s1.equals(s6)
==>
==>
==>
==>
==>
==>
true
false
true
true
true
true
And the following identity results
s5 and s6 refer
to the same
(identical)
literal object.
s1
s1
s1
s1
s5
Java, Java, Java, 3E by R. Morelli | R. Walde
==
==
==
==
==
s2
s3
s4
s5
s6
==>
==>
==>
==>
==>
Copyright 2006. Chapter 7: Strings
false
false
true
false
true
String Identity vs. String Equality (cont)
• In this figure, s1, s2, s4, s5, and s6 are equal.
• Strings s1 and s4 are identical, as are s5 and s6.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Library: java.util.StringTokenizer
• Break up a string into its
tokens -- e.g., breaking up
a name and password pair
in boyd:14irXp.
• The StringTokenizer class
is designed for this
purpose.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
StringTokenizer (cont)
• A StringTokenizer breaks a string into tokens separated by
delimiters, which by default value are the whitespace
characters:
StringTokenizer sTokenizer
= new StringTokenizer("This is an English sentence.");
In this case, the period is
part of last token
Tokens
This
is
an
English
sentence.
• The delimiters can be specified as a String parameter:
StringTokenizer sTokenizer
= new StringTokenizer("http://troy.trincoll.edu/~jjj/index.html”, ":/");
Tokens
Java, Java, Java, 3E by R. Morelli | R. Walde
http
troy.trincoll.edu
~jjj
index.html
Copyright 2006. Chapter 7: Strings
StringTokenizer (cont)
• A while loop can be used to process each token in the
tokenizer:
String url = "http://troy.trincoll.edu/~jjj/index.html";
StringTokenizer sTokenizer
= new StringTokenizer(url, ":/"); // Initializer
while (sTokenizer.hasMoreTokens()) {
System.out.println(sTokenizer.nextToken());
}
// Loop entry condition
// Updater
prints
http
troy.trincoll.edu
~jjj
index.html
• In this example, the StringTokenizer() constructor
initializes loop, the hasMoreTokens() method serves as the
loop entry condition, and the nextToken() method is the
updater. Of course, the StringTokenizer object keeps track
of the tokens in its internal state.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Handling Text in a Graphics Context
• Each graphics context has an associated
Font and FontMetrics object, accessible by:
• A FontMetrics encapsulates font data -e.g., height and width.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
The Font Class
• The Font class provides a platformindependent representation of an individual
font.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
The FontMetrics Class
• The FontMetrics class has methods for
calculating font sizes.
Get the width
of a character
Get the font’s
height.
Or the width
of a string.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Example: Centering a Line of Text
• Each time you resize the window, it picks a
different font and centers “Hello World!”
• Sourcecode/Demo: CenterText.java
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Implementation: CenterText
• For any font, center a string in the window.
No matter what font
the system gives us ...
import java.awt.*;
import javax.swing.*;
public class CenterText extends JFrame {
// Print hello world! in center of frame
public void paint(Graphics g) {
String str = "Hello World!";
g.setFont(new Font("Random", Font.PLAIN, 24)); // Random font
FontMetrics metrics = g.getFontMetrics(); // Get its metrics
Dimension d = getSize();
// Get the frame's size
// Clear the frame
g.setColor(getBackground);
g.fillRect(0,0,d.width,d.height);
g.setColor(Color.black);
// Calculate coordinates
int x = (d.width - metrics.stringWidth(str)) / 2;
int y = (d.height + metrics.getHeight()) / 2;
g.drawString( str, x, y );
// Draw the string
} // paint()
} // CenterText
… use its metrics to
center it.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Technical Terms
baseline
data structure
empty string
lexicographic order
orphan object
string
token
concatenation
delimited string
garbage collection
logical font
physical font
string index
unit indexed
Java, Java, Java, 3E by R. Morelli | R. Walde
copy constructor
delimiter
glyph
off-by-one error
read only
string literal
zero indexed
Copyright 2006. Chapter 7: Strings
Summary Of Important Points
• A String literal is a sequence of 0 or more
characters enclosed within double quote marks. A
String object is a sequence of 0 or more
characters, plus a variety of class and instance
methods and variables.
• The String concatenation operator is the
overloaded + symbol; it is used to combine two
strings into a single String :
“hello” + “world” ==> “helloworld”.
• Strings are indexed starting at 0 (zero indexing).
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Summary Of Important Points (cont)
• The indexOf() and lastIndexOf() methods are
used for finding the first or last occurrence of a
character or substring within a String.
• The valueOf() methods are used to convert a
nonstring into a String.
• The length() method is used to determine the
number of characters in a String.
• The charAt() method is used to return the single
character at a particular index position.
• The various substring() methods are used to
return the substring at particular index positions in
a String.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings
Summary Of Important Points (cont)
• The overloaded equals() method returns true if
two Strings contain the same exact sequence of
characters. The == operator, when used on
Strings, returns true if two references designate
the same String object.
• A StringTokenizer is an object that can be used to
break a String into a collection of tokens separated
by delimiters. The whitespace characters, -- tabs,
blanks, and newlines -- are the default delimiters.
Java, Java, Java, 3E by R. Morelli | R. Walde
Copyright 2006. Chapter 7: Strings